1. Field of the Invention
The present invention relates generally to responding to an attribute of the quantum state of at least one entity, such as a photon or electron, that can be described by a quantum state. More specifically, the present invention relates to systems that are responsive to whether or not the quantum state of an entity is in a superposition of states, and relates as well to applications of these systems including quantum computation, quantum communication, and quantum cryptography, among others.
2. Related Art
In quantum mechanics it is well known that an entity, such as a photon or electron, can exist in a superposition of eigenstates of an observable. When two or more entities share a superposition of states they are “entangled”, so that a particular measurement of the state of one of the entities can also alter the state of the other entangled entities, seemingly even without physical connection between the entities.
The Heisenberg uncertainty relations, which have been well-known since the 1930's, complicate the execution of quantum measurements. In the quantum formalism, observable physical quantities are described mathematically by operators. If two operators A and B are non-commuting (i.e. AB≠BA), then the measurement of either physical quantity represented by A or B will induce an uncertainty in the value of the other physical quantity. Although, in theory, the uncertainty in B induced by the measurement of A does not necessarily affect a subsequent measurement of A, in practice a previous measurement of A will often couple back to A via a “back-action” which can have significant consequences.
Discussions of how to avoid the consequences of back-action effects upon measurements expanded in the 1970's, and a substantial body of research has since developed. These measurement strategies are generally described as quantum nondemolition (QND) measurements. As related in “Quantum non-demolition measurements in optics”, Nature 396, p. 537–542 (1998) by P.Grangier, J. A. Levenson, and J.-P. Poizat on page 537: “The key issue is to devise measurement schemes in which the back-action noise is kept entirely within unwanted observables, without being coupled back onto the quantity of interest. This quantity them remains uncontaminated by the measurement process, allowing repeated measurements to be performed with arbitrary high accuracy”; and as they further relate on page 540: “The main idea in QND strategy is to monitor a single observable that can be measured many times with the same result, identical to the first precise result if no external perturbation is applied.” Others have since broadened their conception of what constitutes a QND measurement into associations with assorted measurement schemes that purport to leave a “quantum” quantity or entity enduring in some manner.